Repeated hammering action is provided in drills for masonry and other hard materials. In one known type of hammer drill, a drill bit is carried in a chuck fixed to a working shaft which is driven via a gear from another shaft, the working shaft carrying the chuck being free to move axially over a small range of distances. A ratchet ring is fixed to the end of the working shaft opposite to the chuck end, and a corresponding ratchet ring is fixed to the body of the tool. One extreme of the allowable axial movement of the working shaft is set by the contact of the two ratchet rings, and this extreme is a function of the angle of rotation of the working shaft. When a user operates the tool, the working shaft is forced backwards such that the two ratchet plates come into contact with each other, and relative rotation of the ratchet rings causes a series of impulses to occur.
Ratchet ring arrangements of this type are relatively inexpensive to construct, but suffer from the drawback that the impulses acting on the working shaft and ultimately passing into the drill bit also have a reaction on the body of the tool, which results in substantial shaking of the tool. A further disadvantage is that friction losses between the two ratchet plates are relatively high.
A further known type of hammer drill which benefits from substantially lower tool body vibration, lower loss of torque at the instant of impact, and more effective impact in most cases because the impulses are generated closer to the drill bit, incorporates a flying striker mass. However, hammer drills of this type require direct axial excitation of the flying striker mass, as a result of which they are expensive to construct.